sendchange with no --revlink creates a Change with revlink=None, not ''

[dustin]

Like most properties, revlink should have a single type.

We're seeing this at Mozilla, and will do more digging.

/tools/buildbot-0.8.2/bin/buildbot sendchange --master localhost:9011 --user ssalbiz --branch users/stage-ffxbld/mozilla-central foo && tail -F twistd.log

2010-11-10 12:11:47-0800 [Broker,15,127.0.0.1] perspective_addChange called
2010-11-10 12:11:47-0800 [Broker,15,127.0.0.1] adding change, who ssalbiz, 1 files, rev=None, branch=users/stage-ffxbld/mozilla-central, repository=, comments , category None, project
2010-11-10 12:11:47-0800 [Broker,15,127.0.0.1] removing change with id 551
2010-11-10 12:11:47-0800 [-] Unhandled Error
       Traceback (most recent call last):
          File "/tools/python-2.6.5/lib/python2.6/threading.py", line 504, in __bootstrap
            self.__bootstrap_inner()
          File "/tools/python-2.6.5/lib/python2.6/threading.py", line 532, in __bootstrap_inner
            self.run()
          File "/tools/python-2.6.5/lib/python2.6/threading.py", line 484, in run
            self.__target(*self.__args, **self.__kwargs)
        --- <exception caught here> ---
          File "/tools/buildbot-0.8.2/lib/python2.6/site-packages/Twisted-10.1.0-py2.6-linux-i686.egg/twisted/python/threadpool.py", line 207, in _worker
            result = context.call(ctx, function, *args, **kwargs)
          File "/tools/buildbot-0.8.2/lib/python2.6/site-packages/Twisted-10.1.0-py2.6-linux-i686.egg/twisted/python/context.py", line 59, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/tools/buildbot-0.8.2/lib/python2.6/site-packages/Twisted-10.1.0-py2.6-linux-i686.egg/twisted/python/context.py", line 37, in callWithContext
            return func(*args,**kw)
          File "/tools/buildbot-0.8.2/lib/python2.6/site-packages/Twisted-10.1.0-py2.6-linux-i686.egg/twisted/enterprise/adbapi.py", line 429, in _runInteraction
            result = interaction(trans, *args, **kw)
          File "/tools/buildbot-0.8.2/lib/python2.6/site-packages/buildbot-0.8.2-py2.6.egg/buildbot/schedulers/base.py", line 146, in classify_changes
            important = self.fileIsImportant(c)
          File "/tools/buildbotcustom2/buildbotcustom/process/release.py", line 85, in <lambda>
            branchConfig['hgurl'])
          File "/tools/buildbotcustom2/buildbotcustom/misc.py", line 116, in isHgPollerTriggered
            if hgUrl in change.revlink or change.comments.find(hgUrl) > -1:
        exceptions.TypeError: argument of type 'NoneType' is not iterable

[Dustin J. Mitchell]

sendchange's --project, --repository, --revlink default to , not None

Fixes #1050

Changeset: 839d230aa10326a3097f467dcb39cd09b3709288